Studies have concerned the regulation of hormonal responsiveness of embryonal carcinoma (EC) cells versus differentiated cell types and the role these changes in hormonal responsiveness might play in regulating differentiation. Initial studies have dealt with the characterization of the adenylate cyclase system of EC cells and F9 cells, a teratocarcinoma cell line that undergoes limited differentiation under normal culture conditions. Of particular interest is the finding that these embryonic cells are quite responsive to calcitonin; this hormonal response is apparently developed just prior to differentiation into endodermal cell types. This raises the possibility that cells develop a calcitonin-sensitive cyclase system to modulate cyclic AMP and calcium levels and in this manner regulate specific cellular processes during development. Other studies have established that F9 cells can be grown under defined, serum-free conditions in the presence of the somatomedin-like peptide MSA, transferrin, and fibronectin. This system should prove useful to study the alteration of responsiveness to, and the mechanism of action of, insulin-like mitogenic factors.